Fed's Williams-Economic, cost factors will be looked at for QE

NEW YORK Feb 21 (Reuters) - A top Federal Reserve official said on Thursday he expects both economic and cost-benefit factors will be taken into account when the U.S. central bank considers adjusting the pace of its current bond purchases.

John Williams, President of the San Francisco Federal Reserve Bank, told reporters he sees tapering bond purchases as making more sense than coming to an abrupt stop early.

Williams also said the Fed's purchases of mortgage-backed securities have proven to be a "very effective tool," and that it is having a powerful effect in the mortgage market.